In an air conditioner in which an air conditioning operation is performed by rotating an electric compressor, noise caused by the rotation of the compressor is likely to result in unpleasant noise for passengers, depending on a vehicle noise level.
For example, if the speed of a vehicle is equal to or higher than a predetermined speed, the vehicle noise level will be high because of road noises and the like. Therefore, the noise caused by the rotation of the compressor will less likely to affect the ear unpleasantly. On the other hand, if the vehicle speed is lower than the predetermined speed, the vehicle noise level will be low. In this case, therefore, the noise caused by the rotation of the compressor is likely to affect the ear unpleasantly.
To solve this problem, in an air conditioner disclosed in JP-A-7-223428, a necessary rotation speed of the electric compressor is calculated and the rotation speed of the compressor is controlled to the necessary rotation speed. Further, when the vehicle speed is equal to or lower than a predetermined level, the necessary rotation speed of the compressor is limited to a predetermined rotation speed even if the necessary rotation speed of the compressor is higher than the predetermined rotation speed.
In this air conditioner, the predetermined rotation speed of the compressor is for example set to 4000 rpm. It is calculated that the necessary rotation speed of the compressor is for example 7000 rpm, which is higher than the predetermined rotation speed. In this case, if the vehicle speed is changed from a level higher than the predetermined level to a level equal to or lower than the predetermined level, the rotation speed of the compressor is reduced from 7000 rpm to 4000 rpm.
On controlling a rotation speed of a motor of the compressor by a compressor rotation speed controlling means such as an inverter, the rotation speed of the motor is generally controlled by detecting a rotational position of a rotor of the motor. However, it takes time to detect the rotational position of the rotor. Therefore, it is difficult to immediately change the rotation speed of the rotor in a shorter time.
In fact, it is delayed to reduce the rotation speed of the compressor from 7000 rpm to 4000 rpm. That is, the rotation speed of the compressor is reduced after the vehicle speed is decreased lower than the predetermined speed and the vehicle noise level is lowered. Accordingly, the noise caused by the compressor is likely to remain, resulting in unpleasant noise for passengers in the compartment.